Module 5 Section 2 (Drugs for Nausea) Flashcards
What is the purpose of vomiting?
Vomiting is a defense mechanism used by the body to get rid of noxious and potentially toxic substances.
- While sometimes vomiting is necessary (i.e. food poisoning).
- Other times vomiting is unnecessary (i.e. motion sickness).
Where are the nausea/vomiting reflexes located?
Nausea and vomiting are reflex responses mediated by the vomiting centre, which is located in the medulla oblongata of the brain.
- The medulla oblongata is a cone shaped mass in the brainstem that is responsible for autonomic functions.
- The ANS regulates involuntary functions in the body.
What is the purpose of the vomiting centre?
The vomiting center receives neural messages from a number of areas in the brain.
What are the various sections of the brain?
1) One section of the vomiting centre, known as the chemoreceptor trigger zone, is rich in dopamine (D2) receptors and serotonin receptors (5-HT3).
2) The chemoreceptor trigger zone is outside of the blood brain barrier and monitors the blood for toxicants and noxious stimuli, and will initiate vomiting in an attempt to rid the body of unwanted substances.
- The chemoreceptor trigger zone also has access to all drugs that are in the circulation, which may explain why so many drugs cause nausea and vomiting.
The vestibular system (a part of the inner ear) can also initiate vomiting.
- This system contains muscarinic (M1) and histaminic (H1) receptors and also sends information to the vomiting center, especially related to motion sickness.
Vagal efferent nerves from the gastrointestinal tract rich in serotonin receptors will send information to the vomiting center regarding irritation from foreign substances.
What are the numerous causes of vomiting?
- gastrointestinal infections and diseases
- food poisoning, over eating, over drinking
- motion sickness, migraine headache, concussions
- brain tumors
- metabolic disease such as hyper or hypoglycemia
- morning sickness in pregnancy (increased estrogens)
- drugs (e.g. cancer chemotherapies)
True or false: understanding the transmitters and receptors involved in vomiting has led to the development and use of several classes of drugs to treat this condition.
True
The drug therapy selected is based on the cause and severity of vomiting.
What are antiemetics?
Drugs used to treat nausea and vomiting.
What are some examples of antiemetics?
1) Antihistamines
2) Serotonin Receptor Antagonists
3) Tetrahydrocannabinol and Derivatives
4) Muscarinic Receptor Antagonist
5) Other drugs for nausea
What are antihistamines and how do they work? Provide an example.
Antihistamines are drugs that antagonize the histamine (H1) receptors in the inner ear and vomiting center, preventing histamine from initiating nausea and vomiting.
- These drugs are not as effective against motion sickness that is already present.
Ex: Dimenhydrinate is sold over the counter as Gravol. It is used extensively as a drug to treat motion sickness.
What are serotonin receptor antagonists and how do they work? Provide an example. What are some adverse effects?
Serotonin receptor antagonists block the effect of serotonin on the serotonin (5-HT3) receptors in the GI tract and vomiting center, preventing stimulation of the vomiting center.
- Serotonin antagonists are the preferred drugs for severe nausea and vomiting associated with cancer chemotherapy, surgery, or radiation.
- These drugs are usually given before chemotherapy or radiation to prevent vomiting.
Adverse effects:
- constipation, diarrhea, and headache.
Ex: ondansetron is a serotonin receptor antagonist marketed under the brand name Zofran.
What are tetrahydrocannabinol and derivatives and how do they work? Provide an example.
Tetrahydrocannabinol (THC), the major psychoactive substance in marijuana, has antiemetic properties.
- These drugs act by stimulating cannabinoid (CB1) receptors in the brain.
- Activation of CB1 receptors inhibits the release of other stimulatory transmitters, thus reducing the stimulatory input to the vomiting center.
Far more effective agents are available, relegating these drugs to second line therapy.
Ex: dronabinol is a purified form of THC sometimes used to treat severe nausea and vomiting caused by cancer chemotherapy.
What are muscarinic receptor antagonist and how do they work? Provide an example.
Muscarinic receptor antagonists block the activity of acetylcholine at its receptor.
- These drugs treat nausea and vomiting by blocking the cholinergic input from the inner ear and the chemoreceptor trigger zone to the vomiting center.
Ex: scopolamine is commonly used in palliative care and can be applied as a patch.
What are some other drugs used to treat nasea?
1) Dopamine receptor antagonists: these drugs, such as the phenothiazines, block the dopamine-mediated input to the vomiting centre from the chemoreceptor trigger zone.
- This class of drugs may be useful in some cases of vomiting due to their sedating properties.
- An adverse effect of the dopamine antagonists is the induction of Parkinson-like symptoms.
2) Glucocorticoids: these are effective drugs for vomiting as additional therapy with other drugs.
- They appear to increase the effectiveness of serotonin receptor antagonists and similar drugs.
Which one of the antiemetics listed is more frequently used in chemotherapy for adults to reduce severe nausea and vomiting?
a) Serotonin receptor antagonists
b) Antihistamine
c) Tetrahydrocannabinol
d) Muscarinic receptor antagonist
a) Serotonin receptor antagonists
Serotonin antagonists (e.g. ondansetron) are the preferred drugs for severe nausea and vomiting associated with cancer chemotherapy, surgery, or radiation. - These drugs are usually given before chemotherapy or radiation to prevent vomiting
